Indirectly heated dispenser cathode

ABSTRACT

A dispenser MK-type cathode structure adapted for storage and/or handling with subsequent activation and a method of producing the same whereby a supply source material of an emission-promoting substance is positioned in an open-ended storage chamber of a plug-like cathode insert member and sealed within the chamber by a fluid impermeable pore-free mask member. A porously sintered tungsten disc member sits above the mask member so as to have an undersurface thereof in fluid communication with the supply source material upon rupture of the mask member. The mask member is sealed to the boundaries of the storage chamber by a selfsticking organic polymer material which is thermally decomposable without residue at operational temperatures of such dispenser cathodes. The mask member comprises a unitary structure composed of a high melting metal foil having at least one surface thereof coated with the organic polymer material, a lamina of a film forming organic material having a layer of the self-thicking organic polymer material along a surface thereof or a self supporting film of the organic polymer material. The mask member is provided in a controlled thickness of sufficient dimension to resist rupture until the environment around the sealed storage chamber is subjected to cathode vacuum conditions.

United States Patent Hoimann et al.

[4 1 Aug. 1,1972

[54] INDIRECTLY HEATED DISPENSER CATHODE [72] Inventors: Horstl-lotmann; Erwin Huebner;

Helrnut Katz, all of Munich, Germany [73] Assignee: SiemensAktlengesellschaft, Berlin and Munich, Germany [22] Filed: Sept. 30,1970 [211 App]. No.: 76,669

[30] Foreign Application Priority Date FOREIGN PATENTS OR APPLICATIONSPrimary Examiner-David Schonbcrg Assistant Examiner-Paul A. SacherAttorney-Hill, Sherman, Meroni, Gross & Simpson 10/ l 968 Canada ..313/346 [57] ABSTRACT A dispenser MK-type cathode structure adapted forstorage and/or handling with subsequent activation and a method ofproducing the same whereby a supply source material of anemission-promoting substance is positioned in an open-ended storagechamber of a plug-like cathode insert member and sealed within thechamber by a fluid impermeable pore-free mask member. A porouslysintered tungsten disc member sits above the mask member so as to havean undersurface thereof in fluid communication with the supply sourcematerial upon rupture of the mask member. The mask member is sealed tothe boundaries of the storage chamber by a self-sticking organic polymermaterial which is thermally decomposable without residue at operationaltemperatures of such dispenser cathodes. The mask member comprises aunitary structure composed of a high melting metal foil having at leastone surface thereof coated with the organic polymer material, a laminaof a film forming organic material having a layer of the self-thickingorganic polymer material along a surface thereof or a self supportingfilm of the organic polymer material. The mask member is provided in acontrolled thickness of sufficient dimension to resist rupture until theenvironment around the sealed storage chamber is subjected to cathodevacuum conditions.

5 Claim, 2 Drawing Figures INDIREC'I'LY HEATED DISPENSER CATI-IODEBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to the dispenser cathodes and more particularly to MK-typeindirectly heated dispenser cathode structures and a method of producingthe same.

2. Prior Art Metal capillary cathodes (i.e., MK cathodes) are known.Generally, such cathodes comprise a confining metal cup member having asupply substance for an emission-promoting material therein (i.e., oneyielding an emission-promoting material, such as barium, at a hightemperature) and a porous disc welded to the rim of the cup member, theupper surface of which defines an emission surface. A heating meansheats both the cup member and the disc causing the supply substance toevaporate and the emission-promoting material to migrate by capillaryaction through the porous discs and to the emission surface. Theemission-promoting material forms a film on the free surface of the discand acts to reduce the work function of the material forming the disc.

In certain more recent developments relating to this type of dispensercathode, modified structures have been prepared wherein relatively thinmetal foils are positioned between the supply materials yielding anemission promoting substance and the porous disc. The supply material issurrounded with an inert atmosphere and the thin metal foil tightlyseals this atmosphere within the storage chamber. In this manner, theassembled dispenser cathode structure can be handled and/or stored witha subsequent activation by rupturing the foil with subjection of theenvironment around the field chamber to cathode vacuum operatingconditions. Such modified structures function relatively reliablyproviding that the thickness of the metal foil is relatively thin.However, it is extremely difficult to economically produce asufficiently thin and pore-free foil of a high melting metal, such astantalum. Further, other high melting metals, such as molybdenum, areextremely difficult to manufacture into foils of precisely controlledthickness.

The instant invention provides a cathode structure and a method ofproducing the same which avoids the aforesaid difficulties and providesnovel masks or foil members of easily controlled thicknesses.

SUMMARY OF THE INVENTION The invention provides a novel dispensercathode structure of the MK indirectly heated type and a method ofproducing the same where a supply material yielding an emissionpromoting substance is stored in an open-ended storage chamber of aplug-like cathode insert member and sealed in an inert atmosphere andfrom the surrounding environment by a fluid impermeable mask membercomposed of a unitary structure including a thermally decomposableorganic polymer material, a combination of a relatively dense highmelting metal foil and a layer of such organic polymer material along atleast one surface of the metal foil and/or a combination of a lamina ofa film-forming organic material and a layer of such organic polymermaterial along one surface of the lamina. In one embodiment, pluralityof such plug-like cathode insert members are spaced about a relativelylarge dimension ring-shaped cathode body and each insert member ismachined to mate or lock with a portion of such body and the mask membersealingly cooperates with the peripheral surfaces of the storage chamberto confine the inert atmosphere and the supply material therein and outof communication with the environment surrounding the chamber, includingthe cathode emission surface. The thickness of the mask member isreadily controlled by liquid or semi-liquid coating techniques. Theorganic material utilized forms a self-sticking film which is readilyunited with a relatively high melting metal foil or an organic materiallamina and readily forms a fluid impermeable seal on the peripheralboundaries of the storage chamber. Additionally, the organic polymermaterial thermally decomposes at normal cathode operating temperaturewithout producing any harmful reaction product or other undesirableresidue.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall diagrammaticsectional view of an embodiment of a ring-shaped cathode structure andincludes a cross-sectional view through an embodiment of an insertmember constructed in accordance with the principles of the invention;and

FIG. 2 is a somewhat enlarged cross-sectional view of another embodimentof an insert member constructed in accordance with the principle of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention provides anindirectly heated dispenser cathode structure of the MK-type. Oneembodiment of such cathode structure includes a ringshaped emissionsurface supported on an appropriately shaped cathode body member whereina supply material (i.e., an alkali earth metal oxide) for anemissionpromoting substance (i.e., an alkali earth metal) is spaced awayfrom the emission surface within the cathode body member and distributedaround the perimeter thereof. The emission-promoting substance migratesor travels from a storage chamber and through a porously sintered covermember positioned above such storage chamber. The upper free surface ofthe porous cover member defines the cathode emission surface and theemission-promoting substance forms a thin film thereat to reduce thework function of such emission surface. Preferably, the porous disc iscomposed of a porously sintered tungsten and the supply source materialis i.e., an alkali earth metal oxide, and particularly barium oxide.

The ring-shaped cathode structures are provided with a relatively largeemission surface composed of a porously sintered tungsten or otheremission base material supported along peripheral portions thereof by abody member. The body member is composed of a relatively high meltingmetal, such as molybdenum, and is provided with a plurality of axialbores extending toward the emission surface so as to be in fluidcommunication therewith. These axial bores are generally uniformlyspaced around the perimeter of the body member. One or more plug-likeinsert members are provided for insertion into a desired number of axialbores. Each insert member is also composed of a relatively high meltingmetal, such as molybdenum, and has an open-ended storage chamber at anupper axial end thereof for containing a desired amount of a supplymaterial for an emission-promoting substance. The insert members areadapted to mechanically mate or lock with the axial bore walls, as bysuitably provided screw threads or the like.

The supply material (i.e., BaO) for an emissionpromoting substance(i.e., Ba) is formed into appropriately sized tablets or the like forease of usage and insertion into the storage chamber of the insertmember. Of course, the supply material can be used in powdered form, ifdesired. A suitable amount of such supply material is placed within astorage chamber, surrounded or blanketed with an inert atmosphere andsealed. Preferably, a dry inert gas is pumped or injected at aboutambient pressures into the partially filled storage chamber so as tocompletely surround and blanket the supply material. There after thechamber is sealed by a mask member so as to maintain the supply materialunder a blanket of such inert gas at such ambient (i.e., atmospheric)pressure. The mask member is provided with a self-sticking coating whichforms a fluid (i.e., water or gas) impervious seal between the upperboundaries of the storage chamber and the mask member independently ofthe cathode body member. In this form, the sealed insert members arestored or handled as desired without danger of any chemical reaction orthe like from occurring with the supply material. Alternatively, theseal insert members are locked into the axial bores within the cathodebody and the assembled unit is stored or handled as desired untilactivation.

Activation or operational condition is achieved by positioning anassembled cathode body member, i.e., one having sealed insert memberslocked in place, into a suitable envelope or tube device or some otherdevice utilizing a cathode, and evacuating the tube device to provide anoperational vacuum state for such device. The inert gas atmosphere atatmospheric pressures sealed within the storage chamber will ruptureand/or destroy mask member and provide a free path of travel for theemission-promoting material to the emission surface of the porouslysintered covered member. The cathode structure is then indirectlyheated, as by an electrically operating heating coil, to the normaloperating temperatures thereof, i.e., between about 800 to l ,300" C.depending upon the characteristics of the material of the structure andthe supply material.

The mask member is a relatively dense foil or film and includes aself-sticking organic polymer material which thermally decomposeswithout any objectionable residue at the operating temperatures of adispenser cathode. The self-sticking organic polymer materials arepreferably selected from the group consisting essentially of paraffin,acetic acid esters, polyacrylic acid esters, phthalic acid esters andmixtures thereof. The mask member comprises a composite film structurecomprised of a combination of a high melting metal (i.e., Ta or Mo) foiland a layer of the self-sticking organic polymer material along at leastone surface of a metal foil; a combination of a lamina of a film-formingorganic material and a layer of the self-sticking organic polymermaterial along a surface of the lamina; and a self-supporting film ofthe self-sticking organic polymer material. The composite film structureis provided in accurately controlled thicknesses, and preferably in athickness of not more than about l20 micrometers (i.e., am). Thethickness is accurately controlled to insure that the mask member willrupture at the operating conditions of a cathode. The self-stickingorganic polymer material is suitably prepared for application by mixingwith a suitable solvent (for example a relatively volatile low molecularweight alcohol) so that the resultant liquid or semi-liquid mixture isamenable to coating techniques and can be applied in a layer of adesired thickness on a base source to provide a substantially pore-freemask member.

In order to more clearly demonstrate one embodiment of a mask memberproduced in accordance with the principle of the invention, thefollowing example is set forth. It will be understood that this exampleis for purposes of demonstration only and does not limit the inventionin any way.

EXAMPLE 1 A self-sticking organic polymer material of the invention isformulated into a self-supporting film suitable for use as a M memberper se by preparing a substantially homogeneous paste-like materialcomprised of about 4.0 milliliters (i.e., ml) of Plexisol P-SSO (aregistered trademark of Roehm & Haas Co. for polyacrylic acid estermaterials), 1.0 ml of butyl acetate and 0.1 gram of phthalic acid ester(phthalic acid ester-bis-Z-ethyl ester) and a suitable solvent. Thispaste-like material is transferred to a draw ring having a slit width ofabout 200 micrometers (i.e., pm) and then drawn through the slit onto asmooth base surface. The solvent used is removed, as by evaporation, anda selfsupporting film is formed which is tacky or selfsticking. Thefinished film has a thickness of about l20 um and forms an excellentmask member which is ruptured at operational cathode vacuum conditionsand is thermally decomposed, without residue, at the operationaltemperatures of indirectly heated dispenser cathode structure.

Referring now to the drawings, it will be noted that FIG. 1 is anoverall diagrammatic sectional view of an indirectly heated dispensercathode structure which has a relatively large diameter ring-shape, andthose elements thereof which do not materially aid a betterunderstanding of the principles of the invention are left out or aremerely not given a reference numeral.

As shown, a ring-shaped cathode body member 1 supports and carries aring-shaped disc member along peripheral portions thereof so that itsupper surface defines an emission surface 20. Preferably, the discmember 2 and the emission surface 24 are composed of the same material,such as porously sintered tungsten. The cathode body member 1 isprovided with a plurality of axial apertures or bores 3, generallyuniformly spaced about the perimeter of body number 1. The axial bores 3extend generally toward the emission surface Za so as to be in fluidcommunication therewith but are somewhat spaced away from disc member 2.An insert member 4 is adapted to mechanically mate or lock with theperipheral walls of the axial bores 3. The insert member 4 is providedwith an open-ended storage chamber 5 opening toward said disc member 2.A passageway means 11 is provided along the upper surface of the bores 3so as to provide uniform communication with the exposed underside of thedisc member 2.

The plug-like insert member 4 is formed somewhat in the shape of ahollow set screw or the like having an open ended storage chamber 5. Asupply material for an emission-promoting substance is pre-formed intotablets 6 and a suitable number of such tablets 6 are positioned withinthe chamber 5 and a dry inert atmosphere is injected within theremaining hollow space of the chamber so as to completely surround orblanket the supply material. The inert atmosphere is maintained at aboutambient pressure, although a slightly positive pressure can be utilizedif desired. Thereafter, the storage chamber 5 is tightly sealed from thesurrounding environment by a mask member 8.

Preferably, the mask member 8 is a composite film structure having arelatively high density and is porefree so as to be impervious to fluidsand particularly water and CO in the embodiment shown at FIG. 1, themask member 8 comprises a lamina 9 of an organic filmforming materialhaving a layer 90 of the self-sticking organic polymer material along asurface of the lamina. As will be appreciated, the lamina 9 may becomposed of the self-sticking organic polymer material per se or ofanother organic film forming material. The assembled unit, such as shownin FIG. 1, can be stored or handled as desired without any danger of achemical reaction occurring with the supply material within chamber 5.When such an assembled unit is inserted into an envelope or tube device(not shown) and the environment surrounding the cathode is evacuated,the gas pressure within the chamber 5 will burst the mask member 8 andprovide a path of communication between the chamber 5 and the adjacentdisc member 2. The passageway 11, which may be somewhat ringshaped,provides fluid communication around the entire periphery of the bodymember I so that the emission-promoting substance can uniformly contactthe entire underside of the disc member 2.

FIG. 2 illustrates another embodiment of the mask member 8 constructedin accordance with the invention. ln this embodiment, the structure isessentially similar to that explained in conjunction with FIG. 1 exceptthat the supply of material is utilized in powdered form 60. The maskmember 8 is composed of a unitary structure having a relatively highmelting metal foil 13 and a layer 9!) of the self-sticking organicpolymer material along a surface of the foil. Of course, it will beappreciated, the metal foil 13 can be sandwiched between a pair of suchlayers 9b, if desired.

Thus, the invention provides a novel indirectly heated dispenser cathodestructure comprising a shaped porous disc member having an emissionsurface with a cathode body member supporting and carrying the discmember along a surface thereof opposed the emission surface. The cathodebody member has a plurality of axial bores distributed about theperimeter of the body member and extending toward the emission surface.A plurality of insert members are provided which are adapted tomechanically lock with the walls of the axial bores and each insertmember has an openended storage chamber extending toward the disc memberwith a supply material for an emission-promoting substance sealed withinthe chamber and surrounded by an inert atmosphere. A fluid impermeablemask member is positioned between the storage chamber and a passagemeans within the cathode body member that provides communication betweenthe storage chamber and the disc member to block travel of theemission-promoting substance to the disc member. The mask member isprovided with a selfsticking organic polymer material along a surfacethereof or sealing the storage chamber independently of the body member.

Various modifications and alterations can, of course, be effectedwithout departing from the novel concepts of the invention.

We claim as our invention:

1. An indirectly heated dispenser cathode structure comprising, a shapedporous disc member having an emission surface, a cathode body memberpositioned to support said disc member along a surface thereof oppositesaid emission surface, said body member having a plurality of axialbores spaced about the periphery thereof and extending toward said discmember and in fluid communication with the supported surface thereof, aplurality of insert members adapted to mechanically lock with peripheralside walls of said axial bores, each of said plurality of inset membershaving an open-ended storage chamber facing toward said disc member, apassageway means positioned within the cathode body member providingfluid communication between said storage chamber and the support surfaceof said disc member, a supply material for an emission-promotingsubstance positioned within said storage chamber and surrounded by aninert atmosphere, and a fluid impermeable mask member positioned onupper peripheral walls of said storage chamber so as to seal theinterior thereof from surrounding environment, said mask memberincluding a self-sticking organic polymer material thermallydecomposable without residue at operating temperatures of a dispensercathode structure.

2. An indirectly heated dispenser cathode structure as defined in claim1 wherein the organic polymer material is selected from the groupconsisting essentially of paraffin, nitro-cellulose, acetic acid esters,polyacrylic acid esters, phthalic acid esters, and mixtures thereof.

3. An indirectly heated dispenser cathode structure as defined in claim1 wherein the mask member comprises a composite structure of arelatively high melting metal foil and a layer of the organic polymermaterial on at least one surface of said metal foil.

4. An indirectly heated dispenser cathode structure as defined in claim1 wherein the mask member comprises a self-supporting film of theorganic polymer material having a thickness of about 1 20 micrometers.

5. An indirectly heated dispenser MK cathode structure comprising, aring-shaped porously sintered tungsten disc member having a free surfacethereof defining an emission surface for such cathode structure, acathode body member positioned to support said disc member along aportion of an undersurface thereof, said cathode body member having aplurality of axial bores substantially uniformly spaced about theperimeter thereof and in fluid communication with said undersurface ofthe disc member, said bores having a crossmask member positioned onupper peripheral boundaries of said storage chamber so as to seal theinterior thereof from fluid communication with the unsupportedundersurfaces of said disc member until the rupture of said mask member,said mask member including a self-sticking organic polymer materialselected from the group consisting essentially of paraffin,nitrocellulose, acetic acid ester. polyacrylic acid ester, phthalic acidester and mixtures thereof.

t t I I i

2. An indirectly heated dispenser cathode structure as defined in claim1 wherein the organic polymer material is selected from the groupconsisting essentially of paraffin, nitro-cellulose, acetic acid esters,polyacrylic acid esters, phthalic acid esters, and mixtures thereof. 3.An indirectly heated dispenser cathode structure as defined in claim 1wherein the mask member comprises a composite structure of a relativelyhigh melting metal foil and a layer of the organic polymer material onat least one surface of said metal foil.
 4. An indirectly heateddispenser cathode structure as defined in claim 1 wherein the maskmember comprises a self-supporting film of the organic polymer materialhaving a thickness of about 120 micrometers.
 5. An indirectly heateddispenser MK cathode structure comprising, a ring-shaped porouslysintered tungsten disc member having a free surface thereof defining anemission surface for such cathode structure, a cathode body memberpositioned to support said disc member along a portion of anundersurface thereof, said cathode body member having a plurality ofaxial bores substantially uniformly spaced about the perimeter thereofand in fluid communication with said undersurface of the disc member,said bores having a cross-axial passageway along the upper boundariesthereof providing fluid communication with each other and with anunsupported portion of the undersurface of said disc member, a pluralityof insert members adapted to mechanically lock with peripheral sidewalls of said axial bores, each of said insert members having anopen-ended storage chamber facing toward said disc member, a supplymaterial for an emission-promoting substance positioned within saidstorage chamber and surrounded by an inert atmosphere at substantiallyambient pressures, and a fluid impermeable pore-free mask memberpositioned on upper peripheral boundaries of said storage chamber so asto seal the interior thereof from fluid communication with theunsupported undersurfaces of said disc member until the rupture of saidmask member, said mask member including a self-sticking organic polymermaterial selected from the group consisting essentially of paraffin,nitrocellulose, acetic acid ester, polyacrylic acid ester, phthalic acidester and mixtures thereof.